stream-chain/src/utils/rreduceStream.js

'use strict';

const {Writable} = require('stream');

const defaultInitial = 0;
const defaultReducer = (acc, value) => value;

const reduceStream = (options, initial) => {
  if (!options || !options.reducer) {
    options = {reducer: options, initial};
  }
  let accumulator = defaultInitial,
    reducer = defaultReducer;
  if (options) {
    'initial' in options && (accumulator = options.initial);
    'reducer' in options && (reducer = options.reducer);
  }

  const stream = new Writable(
    Object.assign({}, {objectMode: true}, options, {
      write(chunk, encoding, callback) {
        const result = reducer.call(this, this.accumulator, chunk);
        if (result && typeof result.then == 'function') {
          result.then(
            value => {
              this.accumulator = value;
              callback(null);
            },
            error => callback(error)
          );
        } else {
          this.accumulator = result;
          callback(null);
        }
      }
    })
  );
  stream.accumulator = accumulator;

  return stream;
};

module.exports = reduceStream;